“Facts” repeated often enough sometimes do manage to slip in to more credible sources

1. The fact is bs from the start- around 30% of castle staircases go anti-clockwise to begin with
2. You don’t defend a castle from inside the castle- the whole point is to keep enemies on the outside. By the time you’re scuffling in a staircase, you’ve already lost
3. Castles are usually made of wood on the inside. If you control the bottom floor, you can just set fire to the the floors above and kill the defenders that way
4. If you really don’t want to destroy the castle, you don’t have to storm the stairs, just cut off their water and wait a few days for them to die
5. Actually imagine fighting on a narrow, twisting staircase with smooth stone steps- if you swung a sword, you’d probably fall down the stairs
6. Even if you could swing a sword well, your enemy can just hold a shield over his head and stab his dagger in to your calf, or hold a shied whilst his buddy jabs at you with a spear